1. Field of the Invention
The present invention relates to an optical concentrator and an optical communication network using the same.
2. Related Background Art
Along with developments of computers and their peripheral devices, a LAN (Local Area Network) for networking the computers and their peripheral devices has been popular. LANs are classified into LANs using an electrical signal, an optical signal, and a radio signal. LANs can also be classified into a data type LAN and a video type LAN in accordance with types of information. A typical data type optical LAN is an FDDI (Fiber Distributed Data Interface). The network configuration of the FDDI is shown in FIG. 1. This configuration is obtained by coupling stations (nodes) through links constituted by optical fiber transmission channels. Stations are classified into a dual attachment station 1711 and single attachment stations 1701, 1702, . . . The dual attachment station constitutes a dual ring using two links. One dual ring 1741 is used for actual data transmission, while the other dual ring 1742 is used during a system failure.
Each single attachment station has only one link. The single attachment stations are connected to concentrators 1721, 1722, 1723, and 1731 capable of connecting a plurality of single attachment stations through upstream and downstream optical fibers 1751 and 1761, thereby constituting a single ring.
The concentrator has a function of arranging single attachment stations in a star shape to increase the number of single attachment stations which perform loop type communication. The schematic arrangement of a concentrator having four ports is shown in FIG. 2. This concentrator has input ports 1811, 1812, 1813, and 1814 to which upstream optical fibers are connected, and output ports 1821, 1822, 1823, and 1824 to which downstream optical fibers are connected. Optical receivers (O/Es) 1831, 1832, 1833, and 1834 convert input optical signals into electrical signals, and optical transmitters (E/Os) 1841, 1842, 1843, and 1844 convert electrical signals into optical signals and output the optical signals. The fourth port (constituted by input and output ports) in FIG. 2 serve as a repeating port. Nodes are respectively connected to the first, second, and third ports through optical fiber transmission channels, respectively. For example, an FDDI optical signal output from the node connected to the first port is input to the input port 1811 and converted into an electrical signal by the optical receiver 1831. This electrical signal is then converted into an optical signal again by the optical transmitter 1842. This optical signal is sent from the output port 1822 to the node connected to the second port. Other optical signals are sent in the same manner as described above. Therefore, the optical signals are sent to another concentrator through the repeating port.
Each concentrator has a repeating function of converting an input optical signal into an electrical signal and then this electrical signal into an optical signal again and outputting the optical signal (this function will be referred to as electrical regenerative repeating or simply regenerative repeating hereinafter). FDDI optical signals are sequentially sent to the nodes. In the FDDI, a signal is transmitted by packet switching or a combination of packet switching and circuit switching. In a node such as a single or dual attachment station, an optical signal is appropriately processed after being converted into an electrical signal, and then the electrical signal is converted into an optical signal again, thereby outputting the resultant optical signal.
On the other hand, a large-capacity communication channel is required for a video type LAN because it processes information having a large volume. For this reason, a low-end video type LAN apparatus which can be used in a general office is not yet developed. However, a broad ISDN (B-ISDN) or the like has been considered as a future video network. In this network, subscribers are connected in a star shape centered on a circuit exchanger or switching unit, and the subscribers can exchange information having a large volume such as video information with each other.
Higher-performance computer systems have been developed, and a high-speed computer network centered on a supercomputer has been recently prepared.
An HIPPI (High Performance Parallel Interface) has recently received a great deal of attention as an input/output interface for a supercomputer, and standardization of the HIPPI is in progress at the ANSI (American National Standard Institute). The HIPPI is an interface for transferring a parallel signal having a 4-byte width at a rate of 100 Mb/s and can be utilized for communication between supercomputers and video transfer. A high-speed network configuration using the HIPPI is shown in FIG. 3. This configuration is obtained by coupling workstations (WSs) and a supercomputer (SC) through access units (AUs) via a link constituted by a transmission channel as of optical fibers. Workstations (WSs) 1901, 1902, and 1903 are connected to access units (AUs) 1922 and 1923 through transmission channels 1941, 1942, and 1943 of a branch LAN. A supercomputer (SC) 1911 is connected to the access unit 1921 through a transmission channel 1951 of the HIPPI standards. The access units 1921, 1922, and 1923 are respectively connected to optical fiber transmission channels 1931, 1932, and 1933 of a very-high-speed backbone LAN. Communication between the workstations and the supercomputer is performed such that the WS 1901 converts an internal parallel signal into a serial or parallel signal of the branch LAN standards and outputs the serial or parallel signal to the AU 1922 through the transmission channel 1943. The AU 1922 converts this signal into a serial signal of the very-high-speed backbone LAND standards and outputs it to the AU 1921 through the optical fiber transmission channel. The AU 1921 converts this signal into a parallel signal of the HIPPI standards to access the SC 1911 through the transmission channel 1951. The SC 1911 transmits a signal processing result to the WS 1901 in procedures reverse to the procedures described above, thereby completing the communication. In this manner, each WS communicates with the SC or another WS.
However, when a workstation or supercomputer processes information having a large volume, the capacity of the backbone LAN is undesirably increased, and the communication circuits of the backbone LAN apparatus or workstation are overloaded. Even if an optical signal is used to increase the communication capacity, the following problem is posed by a loop type optical LAN such as an FDDI. In a station (node) or concentrator, the electrical circuit of the station is overloaded in multi-channel transmission of high-speed signals such as video signals because the optical signal is converted into an electrical signal. In addition, in time division multiplex communication, a circuit switching type optical network must switch a communication route for each packet, thereby complicating the switching unit or exchanger and failing to perform a high-speed operation. In addition, it is difficult to transmit a signal to a plurality of arbitrary subscribers.
In order to solve these problems, a network in which a loop type LAN is arranged integrally with a star type LAN is taken into consideration. The loop type LAN transmits a time division Signal or the like, while the star type LAN transmits a large-capacity signal. In this manner, the circuit is switched in accordance with the types of signals. For example, such a network is proposed in IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 8, NO. 6, PP. 948-963, AUGUST 1990 or the like. FIG. 4 shows the configuration of such a network. This network includes nodes 7611, 7612, . . . 7615, a loop circuit transmission channel 7621, a loop circuit backup transmission channel 7622, and a star circuit transmission channel 7623. A time division signal such as an FDDI signal is transmitted through the loop circuit, and a large-capacity signal such as a video signal is transmitted through the star circuit, thereby compensating for the drawbacks of each other.
However, the above network undesirably has a long total transmission channel and a difficulty in expansion of the network because the transmission channels are independently installed. Dual attachment is generally provided for the loop type transmission channel to overcome failures such as disconnections of transmission channels and failures of node devices. In this case, if failures have occurred in a plurality of locations, the network is undesirably divided.